Electric Steering applications such as Electric Power Steering (EPS), four wheel steering e.g., Quadrasteer™ (Qsteer) and Active Front Steer (AFS) are used in vehicles to improve performance fuel economy and stability of the vehicle. Commonly, in such systems an electronic controller is configured to drive an electric motor to provide torque, velocity or positioning control.
Steering applications such as AFS and Quadrasteer™ utilize motor position control. It is desirable to use a brushless permanent magnet (PM) motor in such applications for its higher efficiency and high torque density. Generally, the motor can be designed and controlled to exhibit a sinusoidal back EMF (electromotive force), which provides smoother torque feel or a trapezoidal back EMF, which while easier to control, can suffer from commutation ripple and noise. The sinusoidal back EMF motor can be controlled utilizing phase advance, thus further reducing the size of the motor. Therefore, it is often desirable to use brushless permanent magnet motors with sinusoidal back EMF for these applications. Brushless permanent magnet motors can be position controlled employing either current mode control or voltage mode control. Voltage mode control advantageously, provides damping when applied voltage is not compensated for back EMF. In voltage mode control, the voltage command to the motor is primarily a function of application control variables disregarding the motor characteristics. In addition, voltage mode control systems may be desirable in certain applications because the need for external sensors to provide feedback is minimized. Unfortunately, however, with voltage mode control the torque, and therefore, the current flowing through the motor is not measured or controlled. For position control applications using voltage command, the voltage is a direct function of the position error, therefore, a high voltage is applied across the motor winding at high position errors while a small voltage is applied for small position motor irrespective of motor velocity. By the principal of the operation of the motor, the voltage applied across the motor is function of motor torque and the back EMF of the motor. At very low velocity, even small voltage applied across the motor can result into high torques and therefore high current. At higher voltage and low velocity, the torque, and thereby the current of the motor can be several times a motors rating. Steering control systems employing voltage mode control algorithms, generally do not use the motor phase current for torque control. Moreover, it may be beneficial to limit motor torque and current to avoid exceeding motor or controller ratings. While the phase current is readily available for measurement, such measurement would require additional sensors and interfaces. Therefore, in a voltage control system, it may be desirable to constrain maximum voltages and thereby avoid exceeding rated torques and currents without relying upon current sensors or measurements.